The choice and dosing of opioids has thus far been largely empirical, with a frequent need to switch opioids and alter doses due to inherent differences in individuals. By this study, the investigators hope to determine the 'right' doses of the 'right' opioids for optimized and safe postoperative analgesia in children. Given the unexplored nature of this complex phenomenon, The investigators propose a comprehensive study to evaluate gene, drug, and surgical interactions, in a large sample of 650 children undergoing different surgeries, requiring patient controlled analgesia with opioids.